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3.3 Laser scanning for height measurements 
In the Netherlands and Belgium the coastal zone managers are 
considering the introduction of laser technology to derive height 
information for the beach and coastal dune area. This new 
technique will provide the necessary data sets for coastal zone 
management, aiming both at the reduction of costs and the 
improvement of the products. Laser scanning will provide an 
elevation model of the sandy coast. From the resulting digital 
elevation model (DEM) profiles of the beach and foredunes will 
be extracted on a yearly basis, replacing the costly and time 
consuming determination of profiles through photogrammetric 
means. At the same time the laser scanning data, covering an area 
in total, provides an opportunity to assess more accurately the 
volumes of sand having been eroded from beach and foredunes. 
For the inner dune area the elevation data will allow changes to be 
monitored, for example with respect to erosion and accretion of 
sand and to study morphological processes. However, in many 
parts the construction of an accurate model of the ground surface 
elevation is hampered by dense vegetation cover. The main 
objective is therefore to provide a general overview of the area, of 
benefit to the manager. To this end the laser DEM will be 
combined with imagery providing thematic information, such as 
land cover. The imagery will be obtained from airborne digital 
video. Main consideration for the selection of video, among 
alternatives like aerial photography and optical line scanners, is 
the relative by low cost. However, the savings realised with 
respect to the imagery might be annulled by the increase of cost 
for the further processing of the video data (e.g. geometric 
correction and mosaicing) and should be further investigated. 
Another disadvantage is the relative by low resolution of the 
video imagery in comparison to aerial photography. The user will 
decide in this respect whether video imagery satisfies his 
demands. 
The laser DEM and video data will be placed in a GIS en- 
vironment with specific tools to allow the user to integrate depth 
and height measurements of the coast, extract cross sections of the 
coastal zone, assess the quality of elevation models derived and to 
generate bird's eye views of areas one is interested in. 
In 1995 a number of experiments were performed to assess the 
accuracy and reliability of the laser technique for DEM 
acquisition as well as to gain insight in its operational aspects. 
Large project areas were appointed in several parts in the 
Netherlands along the coast and estuaries, the most important 
being the group of Frisian Islands in the north. The total area 
involved several hundreds of square kilometres. The quality 
demands were specified as following: 
- point density 1 per 16 m^; 
-  amaximum systematic error of 5 cm; 
- a precision of 15 cm (standard deviation) for non-covered or 
sparsely vegetated areas; 
- a precision of 20 cm (standard deviation) for areas covered 
with dense vegetation; 
In every project area, small patches were chosen where control 
measurements were carried out, both for checking on point 
density as well as assessing the geometric quality by simple 
statistical means. Furthermore, laser derived coastal profiles were 
compared with the photogrammetric derived coastal profiles. In 
figure 3 the results of such a comparison are presented, made 
between 132 photogrammetrically surveyed coastal profiles of the 
island of Ameland, and the laser profiles. The latter are generated 
by interpolation from a regular grid of laser points to the 
planimetric co-ordinates of the profile points. For each profile 
differences are then calculated between the interpolated laser 
heights and the corresponding photogrammetric heights. From 
these differences, a systematic offset and a RMS error can be 
calculated for each of the 132 profiles. In figure 3, the X-axis 
gives the position of a profile along the coast, in kilometres from 
the most western point of Ameland. On the Y-axis, the systematic 
offset and the RMS error are given in meters above Mean Sea 
Level. The RMS error is generally about 15 cm in magnitude. The 
systematic offset starts low in the west (between kilometre 1 and 
3), then suddenly increases to 15-20 cm. 
The results turned out to be slightly disappointing, as in general 
the quality demands were not met. Systematic offsets larger than 
10 cm occurred, together with RMS errors of more than 20 cm in 
magnitude. For assessing sand volumes at the beach, these are 
unacceptable figures. Also, many parts of the delivered DEMs 
showed the presence of remaining large outliers, that had not been 
filtered out, due to a too inflexible filtering method applied and 
the complexity of the terrain topography, which the laser scanner 
could not handle properly. 
  
  
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Distance along coast line (km) 
Figure 3: Differences between photogrammetrically surveyed coastal profiles and laser profiles 
15 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996